System and method for dynamically swapping master and slave PHYs to allow asymmetry in energy efficient ethernet

ABSTRACT

A system and method for dynamically swapping master and slave physical layer devices (PHYs) in energy efficient Ethernet (EEE). A physical layer communication mechanism can be used to dynamically reassign the master/slave assignments to facilitate the asymmetric application of EEE to a link.

This application claims priority to provisional application No.61/033,613, filed Mar. 4, 2008, and provisional application No.61/037,152, filed Mar. 17, 2008. Each above-identified application isincorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates generally to Ethernet systems and, moreparticularly, to a system and method for dynamically swapping master andslave physical layer devices (PHYs) in energy efficient Ethernet (EEE).

2. Introduction

Energy costs continue to escalate in a trend that has accelerated inrecent years. Such being the case, various industries have becomeincreasingly sensitive to the impact of those rising costs. One areathat has drawn increasing scrutiny is the IT infrastructure. Manycompanies are now looking at their IT systems' power usage to determinewhether the energy costs can be reduced. For this reason, an industryfocus on energy efficient networks has arisen to address the risingcosts of IT equipment usage as a whole (i.e., PCs, displays, printers,servers, network equipment, etc.).

In designing an energy efficient solution, one of the considerations isthe traffic profile on the network link. For example, many network linksare typically in an idle state between sporadic bursts of data, while inother network links, there can be regular or intermittent low-bandwidthtraffic, with bursts of high-bandwidth traffic. An additionalconsideration for an energy efficient solution is the extent to whichthe traffic is sensitive to buffering and latency. For example, sometraffic patterns (e.g., HPC cluster or high-end 24-hr data center) arevery sensitive to latency such that buffering would be problematic. Inother links, there may be some sustained traffic that is at a fractionof the full rate. Examples of this situation include higher end-offloadcontrollers, audio video bridging (AVB) enabled switches/networks thatcan carry full uncompressed HD traffic but can transition to streams ofcompressed traffic, aggregation devices, a link between a phone and aswitch with VoIP traffic running, etc. For these and other reasons,applying energy efficient concepts to different traffic profiles wouldlead to different solutions. These varied solutions can therefore seekto adapt the link, link rate, and layers above the link to an optimalsolution based on various energy costs and impact on traffic, whichitself is dependent on the application.

One solution to addressing low link utilization is to reduce the highdata capacity when it is not needed, thereby saving energy. In otherwords, a link can use a high data rate when data transmission needs arehigh, and use a low data rate when data transmission needs are low. Inanother solution, the link can be designed to enter into a low poweridle (LPI) mode where the bulk of the PHY and the energy on the link isturned off (put to sleep) when there is no data transmission. When datais transmitted, it is transmitted at full PHY capacity. While these andother solutions have been proposed, what is needed is a mechanism thatenables asymmetry in EEE PHYs reliably without any degradation inbit-error rate (BER), any corruption of packets and/or a restart of thelink.

SUMMARY

A system and/or method for dynamically swapping master and slavephysical layer devices in energy efficient Ethernet, substantially asshown in and/or described in connection with at least one of thefigures, as set forth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features of the invention can be obtained, a moreparticular description of the invention briefly described above will berendered by reference to specific embodiments thereof which areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered limiting of its scope, the invention will be describedand explained with additional specificity and detail through the use ofthe accompanying drawings in which:

FIG. 1 illustrates an example of loop timing.

FIG. 2 illustrates an example of master/slave physical layer devices.

FIG. 3 illustrates an example of master/slave switching.

FIG. 4 illustrates a flowchart of a process of the present invention.

DETAILED DESCRIPTION

Various embodiments of the invention are discussed in detail below.While specific implementations are discussed, it should be understoodthat this is done for illustration purposes only. A person skilled inthe relevant art will recognize that other components and configurationsmay be used without parting from the spirit and scope of the invention.

Ethernet has become an increasingly pervasive technology that has beenapplied in various contexts such as twisted pair, backplane, optical,etc. IEEE 802.3az Energy Efficient Ethernet (EEE) continues to evaluatevarious methods for reducing energy used by reducing link rates duringperiods of low link utilization. In this process, a protocol would bedefined that would facilitate transition to and from lower powerconsumption modes in response to changes in network demand.

In general, a reduction in link rate to a sub-rate of the main rateenables a reduction in power, thereby leading to energy savings. In oneexample, this sub-rate can be a zero rate, which produces maximum powersavings.

One example of subrating is through the use of a subset PHY technique.In this subset PHY technique, a low link utilization period can beaccommodated by transitioning the PHY to a lower link rate that isenabled by a subset of the parent PHY. In one embodiment, the subset PHYtechnique is enabled by turning off portions of the parent PHY to enableoperation at a lower or subset rate. For example, a subset 1 G PHY canbe created from a parent 10 GBASE-T PHY by a process that turns offthree of the four channels. In another embodiment, the subset PHYtechnique is enabled by slowing down the clock rate of a parent PHY. Forexample, a parent PHY having an enhanced core that can be slowed downand sped up by a frequency multiple can be slowed down by a factor of 10during low link utilization, then sped up by a factor of 10 when a burstof data is received. In this example of a factor of 10, a 10 G enhancedcore can be transitioned down to a 1 G link rate when idle, and spedback up to a 10 G link rate when data is to be transmitted.

Another example of subrating is through the use of a low power idle(LPI) technique. In general, LPI relies on turning the active channelsilent when there is nothing to transmit. Energy is thereby saved whenthe link is off. Refresh signals can be sent periodically to enablewakeup from the sleep mode. In one embodiment, a sync signal can be usedon the interfaces (i.e., medium dependent interface (MDI) and PHY/mediumaccess control (MAC) interface) to allow for a quick wake-up from thesleep mode and maintain frequency lock. For example, on the MDIinterface for a 10 GBASE-T signal, a simple PAM2 pseudorandom bitsequence could be used on pair A during LPI mode. This would notsignificantly increase the power that is consumed.

In conventional applications, low link utilization is addressed on thelink as a whole. This conventional application would not properlyaddress those network situations that are characterized by asymmetrictraffic loads. For example, in those networks that include a videoserver (or switch connected to a video server) there may be a desire toreduce the link capacity in only one direction. This asymmetric linkcapacity reduction would still serve to reduce the amount of power beingconsumed.

A full duplex channel carries point-to-point signaling in bothdirections simultaneously. Echo cancellation is used to separate aremote transmitter's signal from the local transmitter's signal. Inmulti-pair operation, crosstalk cancellation is used to remove otherlocal transmitter's signals from the remote transmitter's signal. Mostforms of echo and crosstalk cancellation require the local transmitterand local receiver be driven from the same clock source to derive theirsampling.

A conventional mechanism of achieving the proper clocking in apoint-to-point link is by employing loop timing. Loop timing ensuresthat both ends of a point-to-point link are in exact frequency lock. Anexample of loop timing is illustrated in FIG. 1, where one side of thelink derives its timing from free running local oscillator 110. Thisside of the link is commonly called the master.

The other side of the link, which is commonly referred to as the slave,derives its timing from a recovered clock. The slave's clock isrecovered from the master's transmitted signaling using phase lockedloop (PLL) 120 causing the slave to be frequency locked to the master.Loop timing is used in various Ethernet standards (e.g., IEEE 802.3 1000BASE-T and 10 GBASE-T) for symmetrical data transport between two PHYsin a point-to-point configuration.

In an Ethernet link, a PHY can be configured to transmit full rate idlesymbols during periods where there is no data to transmit. There islittle or no difference in the physical signaling on the line resultingfrom idle symbols and data symbols. PHY power dissipation whiletransmitting and receiving idle symbols is typically similar (or equalto) PHY power dissipation while transmitting and receiving data symbols.

As noted, some point-to-point applications include asymmetric trafficloads where data traffic in one direction is significantly higher thandata traffic in the other direction. In such a case, one direction willcontain mostly data while the other direction will contain mostly idlesymbols. Idle symbols are used to maintain loop timing and adaptive DSPelements. However, since no information is transmitted with idlesymbols, the power required to keep the lightly utilized directionactive with idle symbols is effectively wasted. Rather than transmittinglong periods of idle symbols, it would be advantageous to turn off thetransmitter on the end of the link, which would otherwise be wastingpower transmitting idle symbols. It would also be advantageous to turnoff the receiver on the end of the link, which would otherwise bewasting power receiving idle symbols.

In a link where one direction has been powered down or otherwise reducedwhile the other direction continues to carry traffic (referred to as an“outage interval”), a mechanism is needed to power up the unuseddirection when data transmission (or refresh intervals) resumes.Ideally, this process would not disrupt the ongoing data communicationin the direction that was not powered down or otherwise reduced.

One of the important functions to be restored to permit powering up thefull duplex link is loop timing so that echo and crosstalk cancellationcan resume. To avoid disrupting ongoing data communication, the looptiming is established immediately, without the need for re-acquisition,or is maintained during the outage interval.

If data communication is powered down in the direction going from slaveto master, the slave transmitter and master receiver are powered downduring the outage interval. In this case, the master transmitter andslave receiver remain powered-up communicating at the full rate. Asillustrated in FIG. 1, loop timing is maintained in this case. Datatransmission from the slave to the master can be powered off withoutdisrupting loop timing. If data communication is powered down in thedirection going from the master to the slave, then the mastertransmitter and slave receiver are powered down during the outageinterval. As illustrated, loop timing is lost during the outage intervalin this case. Data transmission from the master to the slave cannot bepowered off without risk of losing the ability to instantaneouslyestablish loop timing upon resumption of data communication.

It is generally not known a priori which direction in a full duplex linkmay have long intervals of idle transmission. A mechanism that permitsestablishment or maintenance of loop timing with an outage interval ineither direction is therefore needed. Conventionally, PHYs can have thecapability of implementing either the master side or the slave side of apoint-to-point full duplex link. As illustrated in FIG. 2, PHYs have theability to switch roles as the master or the slave. The assignment ofmaster or slave is generally decided randomly during autonegotiation.When the assignment is random, either side can arbitrarily become masteror slave.

At any given instant in a master-slave loop timed link, the frequency ofthe two PHYs are precisely equal to each other. Thus, the assignment ofmaster or slave can be reversed at any given instant in time withoutdisrupting the loop timing operation. The slave side can become themaster by switching to a free running oscillator using the giveninstantaneous frequency offset. The current instantaneous frequencyoffset is available while a PHY is in slave mode. At the same instant,the master side can become the slave by switching to a PLL that tracksthe frequency of the received signal.

When it is desired to power down data communication in the directionwhere the master transmitter is required to power down, the link canfirst swap the definition of master-slave assignment. The newly definedslave transmitter is then powered down and the newly defined masterreceiver is powered down. The newly defined master transmitter and newlydefined slave receiver then maintain data transmission and also maintainloop timing through the outage interval.

To further illustrate this feature of the present invention, referenceis now made to the switching illustration of FIG. 3 and the flowchart ofFIG. 4. As illustrated in FIG. 4, the process begins where an EEEcontrol policy indicates a need for an active master transmitter andslave receiver on a link to enter an outage interval. This scenario ofan active master transmitter and slave receiver is represented by theillustration of FIG. 2.

To facilitate such an indicating need, the process then continues tostep 404 where the master-slave assignments are switched between the twoends of the link. As FIG. 3 illustrates, switching the master-slaverelationship can be facilitated by properly controlling switches 310,320 in the PHYs shown in FIG. 2. More specifically, switch 310 isswitched from a master (M) position to a slave (S) position, therebytransitioning the PHY from an “old master” to a “new slave.” Similarly,switch 320 is switched from a slave (S) position to a master (M)position, thereby transitioning the PHY from an “old slave” to a “newmaster.” In this switching process, the newly defined master can use thefrequency offset available at the instant it switches from slave tomaster, while the newly defined slave can be configured to acquire thefrequency offset. Finally, at step 406, the “old” master transmitter andslave receiver can enter an outage interval, thereby conserving power.As has been described, dynamically swapping master-slave definitionspermits asymmetric outage intervals to be defined in either direction.

In one embodiment, the dynamic reassignment of the master/slaverelationship can be facilitated through a control communicationmechanism that is implemented in the physical layer. For example,sequence or ∥Q∥ ordered sets can be used to communicate controlinformation in switching the master/slave assignments. In general,sequence or ∥Q∥ ordered sets can be used to signal that a reassignmentof the master/slave relationship is needed. In various embodiments, thissignaling can occur prior to otherwise contemporaneous with theasymmetric application of EEE to the link. As would be appreciated, thespecific form of signaling (e.g., in-band, out-of-band, etc.) would beimplementation dependent. For example, 10 G applications can use anout-of-band channel. In another example, a packet-based protocol can bedefined that would enable the periodic exchange of offset information.

As has been described, the swapping of master and slave PHYs enableasymmetry in EEE. In the situation where both sides enter into an outageinterval, the side that comes out of the outage interval first canassume the master role. This can be true whether or not that side washeld the master role regardless master when it entered the outageinterval. In general, it should be noted that while the swapping ofmaster/slave was shown in the context of loop timing, the principles ofthe present invention can be applied to the swapping of master/slave forany function that is shared across the link.

It should also be noted that the principles of the present invention canbe broadly applied to various contexts, such as in all PHYs thatimplement EEE (e.g., backplane, twisted pair, optical, etc.). Moreover,the principles of the present invention can be applied to standard ornon-standard (e.g., 2.5 G, 5 G, etc.) link rates, as well as future linkrates (e.g., 40 G, 100 G, etc.).

These and other aspects of the present invention will become apparent tothose skilled in the art by a review of the preceding detaileddescription. Although a number of salient features of the presentinvention have been described above, the invention is capable of otherembodiments and of being practiced and carried out in various ways thatwould be apparent to one of ordinary skill in the art after reading thedisclosed invention, therefore the above description should not beconsidered to be exclusive of these other embodiments. Also, it is to beunderstood that the phraseology and terminology employed herein are forthe purposes of description and should not be regarded as limiting.

1. An energy efficient Ethernet method, comprising: communicatingbetween a first physical layer device and a second physical layer devicevia a network cable, said first physical layer device operating in amaster mode and said second physical layer device operating in a slavemode, wherein a physical layer device operating in said slave moderecovers a clock from a received signal; after a determination that afirst direction of transmission between a transmitter in said firstphysical layer device and a receiver in said second physical layerdevice should enter an outage interval, transmitting a control messagefrom said first physical layer device to said second physical layerdevice, wherein during said outage interval a second direction oftransmission between a transmitter in said second physical layer deviceand a receiver in said first physical layer device remains operative;switching said first physical layer device from said master mode to saidslave mode, wherein said slave mode is responsive to said secondphysical layer device that has switched from said slave mode to saidmaster mode in response to said transmitted control message; andfollowing said switching, transitioning said first direction oftransmission into an outage interval.
 2. The method of claim 1, furthercomprising transmitting an in-band control message.
 3. The method ofclaim 1, further comprising transmitting an out-of-band control message.4. The method of claim 1, wherein said switching comprises activating aphase locked loop for use by said receiver in said first physical layerdevice.
 5. The method of claim 1, wherein said transitioning comprisesentering a subset physical layer device mode.
 6. The method of claim 1,wherein said transitioning comprises entering a low power idle mode. 7.An energy efficient Ethernet method, comprising: operating a firstphysical layer device as a master physical layer device; determiningwhether a link rate of a first direction of a network cable link betweensaid first physical layer device and a second physical layer device isto be reduced; and if it is determined that said link rate of said firstdirection of said network cable link is to be reduced, then transmittinga control message from said first physical layer device to said secondphysical layer device that is operating as a slave physical layerdevice, said control message requesting a reassignment of said secondphysical layer device from a slave physical layer device to a masterphysical layer device; and after said second physical layer device isreassigned as a master physical layer device, then reducing a link rateof said first direction of said network cable, while a link rate of asecond direction of said network cable link between said first physicallayer device and said second physical layer device is not reduced. 8.The method of claim 7, wherein said transmitting comprises transmittingusing physical layer communication.
 9. The method of claim 7, whereinsaid transmitting comprises transmitting using sequence ordered sets.